Various devices exist that can detect the presence of ice, but that cannot measure its thickness. One such device is described in U.S. Pat. No. 3,277,459 to Werner, issued Oct. 4, 1966. Various other devices are known that can sense or measure the level of a liquid. Examples include the devices described in U.S. Pat. Nos. 4,165,641 to Pomeratz et al, issued Aug. 28, 1979; 4,182,176 to Playfoot et al, issued Jan. 8, 1980; and 4,229,972 to Phillips et al, issued Oct. 28, 1980. Such devices, however, are often temperature sensitive and cannot accurately respond with fluctuating temperatures often present in sea water containing ice. Other devices can measure levels only of paramagnetic liquids, and not of sea water. An example of this type of device is disclosed in U.S. Pat. No. 3,948,100 to Parris et al, issued Apr. 6, 1976.
The liquid level measuring devices that have been most promising for use in measuring the thickness of floating ice are those employing resistors. An example is the liquid level measuring device described in U.S. Pat. No. 3,735,638 to Miller, issued May 29, 1973. A method disclosing the use of the electrical circuit of that device (and other devices with similar resistors) in a probe for measuring the thickness of ice was disclosed in U.S. Pat. No. 4,287,472 to Pan et al, issued Sept. 1, 1981. That method involves embedding a resistor in ice floating on sea water with part of the resistor extending through the ice into the sea water, measuring the electric resistance of the resistor, and relating this measurement to the ice thickness. The sea water, because of its low resistance or high conductivity relative to ice, reduces the effective length of the circuit and thereby decreases its overall resistance. The thickness of the ice is determined by correlating the measured resistance of the circuit with predetermined calibrated ice thickness values for the resistor.
With resistance devices, calibration is critical because a number of extraneous factors may affect the resistance measurements. One such factor is the salinity of the sea water. The salinity affects the resistivity and conductivity of the sea water, and hence affects the overall resistance of a resistor protruding into the sea water. The salinity of sea water, especially sea water containing ice, varies with time and place. Thus, re-calibration is necessary for each location having a different salinity from the previous location. Likewise, repeated calibration is necessary in the same location when the salinity varies, as when sea water is fed by a fresh water stream. Such repeated calibration is sometimes not feasible, and occasionally impossible.
A method for measuring the average thickness of ice floating in sea water, independent of the properties of the ice and dependent on the conductivity of the sea water, was suggested in a paper by P. Hoekstra, A. Sartorelli and S. Shinde entitled Low Frequency Methods for Measuring Sea Ice Thickness and presented at the International Workshop on Remote Estimation of Sea Ice Thickness held at St. Johns, Newfoundland in September of 1979. (C-CORE, Memorial University of Newfoundland, Pub. No. 80-5 (May, 1980)). That method employs a "Geonics EM31," a fiber glass boom with two co-planar magnetic dipoles, one mounted at each end, held over the ice. One magnetic dipole transmits current creating a primary field and inducing eddy current flow in the ice and underlying water. The eddy current causes a secondary magnetic field. The other magnetic dipole measures the ratio of these secondary and primary fields. Hoekstra et al. stated that the amount of eddy current flow in the ice and water is approximately proportional to the product of a geometric factor and the conductivity of the ice and water. Since the conductivity of sea ice is about two orders of magnitude less than the conductivity of sea water, Hoekstra et al. stated that the secondary field is proportional to the ice thickness or the height of the dipoles above the sea water. This method is severely limited, however, because it is impracticable for measuring the thickness of ice at a particular point (rather than the thickness of ice over a broader area) and for measuring the thickness of ice over an extended period of time (which is desirable, for example, in monitoring the growth or melting of the ice). Also, the method is affected by changes in salinity of the sea water because the method is dependent on the conductivity of the sea water and salinity affects conductivity.
A need exists for portable, accurate and economical devices and methods which do not require repeated calibration for measuring the thickness of ice sheets floating on bodies of sea water with varying salinity and which are versatile enough to allow measurement of ice thickness over a period of time.